AVS 72 Session TF+AP+EUV-FrM: Atomic Scale Patterning and Processing

The leading edge of today’s semiconductor manufacturing technology relies on extreme ultraviolet (EUV) lithography, utilizing a short 13.5 nm wavelength – a roughly fourteenfold reduction from the 193 nm ArF sources of previous deep ultraviolet (DUV) lithographic technologies. Use of this wavelength has dramatically reduced achievable critical dimensions, with new high-NA EUV tools resolving half-pitches as fine as 8 nm. However, the development of photoresists capable of translating this resolution into reliably patterned features has not kept pace with the breakthroughs in EUV exposure tools, making this material challenge a bottleneck to continued semiconductor scaling.

Conventional organic resists absorb EUV photons inefficiently, compounding stochastic failures at the ultrathin film thicknesses that pitch scaling now demands. The present work incorporates tin in an organometallic resist to address the need for better absorption, while amine organic linkers enable a tunable solubility switch using a metal-aminate bonding motif not yet reported in such resists. Hybrid molecular layer deposition (MLD) is used to deposit this Sn-aminate material one molecular layer at a time through alternating, self-limiting surface reactions between chemical precursors. This vapor-phase deposition technique affords molecular-level control over film thickness and structure, providing smooth and conformal ultrathin films while avoiding the challenges of solvent processing.

This presentation will introduce hybrid MLD procedures for depositing Sn-aminates as a new set of materials using tetrakis(dimethylamino)tin as a tin precursor with ethylenediamine and p-phenylenediamine as amine linkers. The stability of these materials in air and in various developers is reported, providing insight into their solubility switch mechanism and lithographic process optimization. Electron-beam lithography (EBL) is employed as an EUV proxy, and initial results indicate that Sn-aminates act as promising positive-tone e-beam resists in various developers, including HCl, KOH, and TMAH. The effect of deposition temperature on film structure, composition, stability and EBL patterning quality is discussed between 70 °C and 150 °C. Lower temperatures led to higher growth rates and higher organic content in the films, and accelerated film oxidation and decomposition in air. Optimal patterning was found with films deposited at 90 °C using ethylenediamine as the organic linker and developed in 0.02 M KOH, resulting in a dose-to-clear of 12 mC/cm² with a full-pitch of 25 nm. The effect of air-aging and post-exposure baking is also discussed.

Photoresist processes such as photon absorption, electron generation, bond cleavage, crosslinking, and volatile-product formation are key challenges in extreme ultraviolet lithography (EUVL) that need to be understood and resolved. Here, we investigate functional-group-dependent EUV exposure chemistry using simultaneous total electron yield (TEY) and residual gas analysis (RGA) measurements on a library of model polymer photoresists with defined repeat-unit structures, including methacrylate, acid-functional methacrylate, hydroxyl-containing methacrylate, ether, aromatic, heteroaromatic, imide-containing, chemically amplified epoxy, and metal-oxide-infiltrated polymer systems. Principal component analysis (PCA) was applied to the mass spectrometry data from the RGA as an unsupervised dimensionality reduction technique. Firstly, PCA clearly differentiated between low- and high-outgassing photoresists, with high-outgassing photoresists driving the maximum variance in PC1 (24.5 %). Characteristic signals from the PC loading plots were identified and assigned to corresponding photoresists. Two fragment ions, m/z 31 (CH₃O⁺) and m/z 45 (C₂H₅O⁺), were observed during the exposure of polyethylene glycol (PEG) and poly(2-hydroxyethyl methacrylate) (PHEMA). Despite having identical mass-to-charge ratios, they exhibited opposite trends with respect to exposure dose. PEG displayed increasing intensity, whereas PHEMA showed a decrease with increasing exposure dose. This trend was attributed to differences in the scission mechanisms, with PEG undergoing a solubility switch via main-chain scission, while PHEMA undergoes side-chain scission. This allows us to distinguish these two mechanisms by the time-dependent response of characteristic fragment species. We also examined the relationship between TEY and various outgassing species and found that m/z 44, assigned to CO₂, showed a positive correlation (R²=0.99) with TEY in metal-oxide-infiltrated polymer systems, whereas no correlation was observed in PMMA-only resists. Showing that these infiltrated resists exhibit improved electron production and increased outgassing, both of which are critical to improving resist sensitivity. Moving forward, we anticipate our approach will be extremely useful for screening unknown resists and identifying their mechanisms.

Driven by the demand for atomic-scale precision patterning in advanced semiconductor processing, area-selective atomic layer deposition (AS-ALD) has emerged as a paradigm-shifting technique in nanofabrication, leveraging its bottom-up nature to enable spatially selective and precise film growth. In this work, we report a methodology for achieving accelerated deposition of SiO₂ thin films on SiO₂ surfaces while suppressing growth on metal oxide surfaces using a single additive molecule that acts as both catalyst and inhibitor. To overcome the limited selectivity of conventional SiO₂ AS-ALD processes associated with the use of strong oxidizing agents, we introduce a catalytic SiO₂ ALD process. This process employs a Lewis-base catalyst that both promotes SiO₂ growth with H₂O as a mild reactant at temperatures as low as 100°C, and simultaneously inhibits growth on metal oxide surfaces such as Al₂O₃, TiO₂, RuO_x, and CuO_x. Results show that the catalyzed ALD process using amine additives exhibits a mild inherent growth suppression on metal oxide surfaces—but not on SiO₂—suggesting that interactions between the Lewis-base catalyst and metal oxide surfaces during co-injection with the precursor contribute to the retardation of SiO₂ growth. To further enhance the selectivity between SiO₂ and metal oxide substrates, an amine pretreatment was performed prior to ALD to amplify the blocking capability of the same amines as inhibitors. Comparative evaluation of pyridine and triethylamine revealed different inhibition behaviors depending on the underlying metal oxide surface, with triethylamine providing more effective growth inhibition on Al₂O₃ and TiO₂ surfaces than did pyridine. By combining the inherent growth suppression of the catalyzed ALD process with the additional blocking effect of amine pretreatment, enhanced selectivity for SiO₂ deposition on an SiO₂ growth surface and several metal oxide nongrowth surfaces was achieved. This work provides a new strategy for broadening the applicability of SiO₂ AS-ALD by leveraging the dual functionality of amine additives for both catalytic growth and surface inhibition.

Precursor inventory is one of the least directly observed state variables in an ALD delivery system. It is normally inferred from run time, wafer count or recipe demand rather than measured, so the estimate drifts over the life of the source and anomalous draw down remains invisible. Where sensing is fitted it usually detects liquid presence at a few fixed heights using an element immersed in the precursor, which quantises the reading and adds a wetted surface to a corrosive high purity path.

We determine remaining liquid height from outside a stainless steel ampoule by ultrasonic pulse echo. A piezoelectric element coupled to the outer face of the vessel base launches a longitudinal pulse through the wall. The acoustic impedance contrast between steel and a typical precursor is large, so most energy is reflected at the wall, but the transmitted fraction is sufficient to produce a resolvable echo from the liquid vapor interface. The vessel wall itself becomes part of the acoustic path and no feedthrough or wetted surface is introduced.

Extracting height is an underdetermined inverse problem. The echo delay gives the product of propagation velocity and path length, while the quantity of interest is the path length alone. Sound velocity in these liquids is strongly temperature dependent with a coefficient specific to each chemistry, and ampoules operate at temperatures up to 200 C, so a calibration taken at room temperature does not transfer. The chemistry itself may also be unknown to the operator. We show that the velocity can be recovered in situ at operating temperature from the same acquisition, so height is determined with no prior property data for the fluid.

Silicone oil was used as a deliberately unfavorable test case: attenuation is high, the surface echo is correspondingly weak and higher order reverberations within the liquid produce competing arrivals that must be separated. Recovered height agrees with an independent reference to approximately 1 mm with R2 = 0.997, continuously across the full range rather than at discrete heights, and the residual is limited by the reference rather than by the sensing geometry. The result is stable from room temperature to 150 C and reproducible through repeated thermal cycles, with time of flight tracking the velocity change through heating and cooling.

Because the velocity determination responds to the liquid itself rather than to its height, it is also sensitive to changes in composition, suggesting that a single acquisition may report precursor condition alongside inventory.

AlN is a wide-bandgap semiconductor with exceptional thermal conductivity, temperature stability, and piezoelectric properties, making it a promising material for high power electronics and deep ultraviolet optoelectronics in extreme environments. While plasma-enhanced atomic layer deposition (PEALD) of AlN is well-established as a promising back-end-of-line compatible process for passivation, electrical insulation, and thermal management in devices, it typically results in amorphous or polycrystalline films, which have inferior electrical and thermal properties as well as reduced chemical resistances compared to epitaxial AlN. Most reports of epitaxial AlN growth by PEALD involve the use of atomic layer annealing (ALA), in which an Ar plasma exposure is incorporated into each cycle to induce crystallization of the surface.[1] While the effectiveness of ALA is proven, its use also increases cycle duration, which significantly prolongs total process time and can promote impurity incorporation. For these reasons, the growth of epitaxial AlN without ALA is desirable, though this is challenged by the complexity of controlling the plasma properties to achieve suitable growth conditions.

In this work, we demonstrate the growth of 30-50 nm thick epitaxial AlN films on Al₂O₃, GaN, and Ga₂O₃ at 300 °C using PEALD without ALA or thermal annealing. Plasma diagnostics were used to identify favorable conditions which produce ion energy and flux characteristics comparable to those of plasmas used in ALA. The films were characterized using x-ray reflectivity (XRR), high resolution x-ray diffraction (HRXRD), in-plane grazing incidence diffraction (IP-GID), atomic force microscopy (AFM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and optical measurements. The growth per cycle (GPC) and density ranged from 0.90–1.03 Å and 2.998–3.173 g/cm³, respectively, with deposition on GaN resulting in the highest GPC and density. The topography of the films closely replicated that of the underlying material with roughness from 0.25–1 nm by AFM and 0.83–2 nm by XRR, indicating that the deposition was highly conformal. The epitaxial nature of the AlN (i.e., exhibiting a highly ordered crystalline structure with well-defined orientational relationship to the substrate) is confirmed by HRXRD and IP-GID, which show a single orientation with distinct 6-fold azimuthal rotational symmetry. While the AlN films on Al₂O₃ and Ga₂O₃ exhibit significant mosaicity, the AlN films on GaN exhibit low tilt disorder, as evidenced by narrow out-of-plane rocking curves with FWHM= 0.149 degrees (537 arcseconds).

[1] H. Y. Shih et al., Sci. Rep. 7, 39717 (2017)

Developing approaches to stabilize highly dispersed metals on oxide supports represents one of the grand challenges in catalysis. In this context, the (101) surface of TiO₂ anatase is particularly challenging because it lacks surface defects and has a high propensity for metal clustering. We investigate the role of MoO₃ monomeric clusters, pre-deposited on TiO₂(101), in suppressing clustering of adsorbed Rh, using combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Rh deposited directly on bare TiO₂(101) sinters readily at 300 K, forming nm-sized Rh clusters with apparent heights of 3 Å and larger, serving as a reference system. When Rh is instead deposited on a (2×1)-ordered MoO₃/anatase(101) surface at 163 K, STM reveals significantly smaller, highly dispersed Rh species, with apparent heights of 1.8 Å, which remain stable to 450 K. These species exhibit a higher Rh 3d binding energy of 307.8 eV as compared to 307.2 eV of metallic Rh. Only after annealing to 600 K is significant ripening observed, and the cluster sizes approach those observed for Rh on bare TiO₂(101), while the Rh 3d peak shifts toward lower binding energy. The correlated STM and XPS evolution is consistent with increased final-state screening in Rh 3d peak as Rh clusters grow. Our results indicate that modification with MoO₃, combined with low-temperature deposition, kinetically traps and stabilizes highly dispersed Rh species. Ongoing work aims to identify the Mo-O-Rh binding motifs responsible for this stabilization and to determine whether they can stabilize isolated Rh atoms.